Some recent semiconductor devices or integrated circuits require an internal operating voltage VCCINT which is not the same as an external supply voltage VCCEXT (an operating voltage supplied to the device or integrated circuit). Some of these devices, particularly high-density memories or circuits, require a lower internal operating voltage VCCINT because of the scaling down of the device sizes. In order to provide this internal voltage VCCINT, voltage generators have been used which can be trimmed by blowing fuses. Since the generated voltage can be trimmed down, such a converter is sometimes called a "voltage down converter" in the integrated circuit art.
An example of an internal voltage generator with trim capabilities is shown in FIG. 1. A reference voltage generator 400 supplies a reference voltage VREF to a voltage down converter ("VDC") designated by a block 402. VDC 402 provides an output signal on a line 404 to an internal voltage generator designated by a block 406. Generator 406 couples a trim voltage on a trim voltage line 408 back to VDC 402.
The voltage signal on line 404 is supplied to a control electrode of a transistor 410 of internal voltage generator 406 to control a source-drain current through transistor 410. The source of transistor 410 may be coupled to receive the applied supply voltage VCCEXT. The current through transistor 410 is supplied to the other transistor of circuit 406 as shown in FIG. 1 to generate an internal voltage VCCINT available from a node 412. The trim voltage, a voltage used to vary the voltage out of VDC 402, is supplied to VDC 402 from node a 414 coupled to line 408.
To achieve a target internal voltage VCCINT, fuses F1-F4 may be selectively blown. The voltage at node 414 will then change, which will provide a different trim voltage to VDC 402. VDC 402 will output a different voltage to internal voltage generator 406 via line 404 in response to the different trim voltage. This different output voltage will cause the current through transistor 410 to change, which in turn will cause the outputted voltage VCCINT to change.
A problem with this type of generator is that once any of the fuses are blown, the generator cannot be re-trimmed to obtain certain other internal voltage levels. This is particularly a problem in manufacturing semiconductor devices. For example, prior to packaging an integrated circuit, it is subjected to full functional testing and a target internal voltage VCCINT is determined. Following the manufacture of wafers containing many replications of the integrated circuit, each wafer is tested for the desired internal voltage. If the internal voltage is not the desired voltage, the internal voltage generator may be trimmed to obtain that voltage. However, in the process of blowing fuses the desired internal voltage may accidentally be overshot, and the generator may be beyond re-trimming, depending on which fuses were blown. The die is then no good and will not be packaged after dicing, causing the yield rate to decrease.
If the prior art generator achieves the target internal voltage after trimming, the die may be subjected to full functional testing to determine other parameters of the integrated circuit. During this testing it may be determined that the target internal voltage must be adjusted, for example, due to the integrated circuit's sensitivity to that internal voltage VCCINT. The internal voltage VCCINT may not be trimmed, however, depending on which fuses were blown. Thus, the device may not perform optimally.
Therefore, it is a general object of the present invention to provide an improved voltage generator which overcomes the above-mentioned problems.
Another object of the present invention is to allow testing of all levels of the internal voltage VCCINT for each individual circuit.
A further object of the present invention is to provide a fuse programmable voltage down converter requiring a low chip area.